Net: honor the scope argument

[dylanpulver]

Description

Net.__init__ takes a scope argument and never reads it. The form printed in Net's own docstring returns an unscoped object:

>>> Net("224.0.0.1", scope=conf.iface).scope
None            # expected 'eth0'
>>> Net("224.0.0.1%eth0").scope
'eth0'

__iter__ copies self.scope onto every address it yields, so a Net built with scope= yields plain addresses and packets built from it leave through the default interface rather than the requested one. Net6 inherits __init__, so IPv6 behaves the same way.

The argument arrived with scope support in #4461. That commit wired scope into the signature, into __iter__, into __repr__ and into __hash__, but not into the body of __init__. This patch hands it to ScopedIP, which does the interface resolution and keeps the existing precedence where an inline % overrides the argument.

No test could have caught this. Net.__eq__ does not look at scope, so Net("224.0.0.1", scope=iface) == Net("224.0.0.1%iface") holds whether or not the argument works. The new test asserts on .scope itself for that reason, across the mask form, the start/stop form and the addresses a Net generates.

One related thing I did not touch: __hash__ includes scope but __eq__ does not, so Net("224.0.0.1%eth0") and Net("224.0.0.1") compare equal with different hashes, which makes them distinct keys in a dict. Fixing that shifts comparison semantics, so it seemed worth a separate discussion.